Lap dissolve slide projector

ABSTRACT

A lap dissolve slide projector arranged to receive and hold a slide at each of two projecting stations has polarizing means including a polarizing beam splitter and is operable to selectively project the slide loaded in either one of the projecting stations while previewing the slide loaded in the other. The projector utilizes the polarizing means to control which slide image is being projected and previewed and to effect the lap dissolve transition from the image of one slide to another.

United States Patent 1 14 1 Feb. 19, 1974 Fournie [54] LAP DISSOLVE SLIDE PROJECTOR [76] Inventor: Thomas A. Fournie, 6805 Zumirez pmflary Examl7er Lou1s pnllce Dr, Malibu Calif. 90265 Assistant ExammerA. J. Mirabito PP N05 359,856 A lap dissolve slide projector arranged to receive and hold a slide at each of two projecting stations has po- [521 US CL IIIIIIIIIIII II I I I III 353/83 353/20 larizing means including a polarizing beam splitter and a 51] Int. Cl. 6051) 23/16 is "Parable to selectively project the slide loaded [58] Field 0 Search 353/20 2 81 83 86 either one of the projecting stations while previewing the slide loaded in the other. The projector utilizes the [56] References Cited polarizing means to control which slide image is being projected and previewed and to effect the lap dissolve v UNITED STATES PATENTS transition from the image of one slide to another. 3,215,036 11/1965 Kirkcommell 353/20 a 3,514,198 5 1970 Schacht 353/98 13 Clalms, 1 Drawing Flgllre IV 27!? I LAP DISSOLVE SLIDE PROJECTOR BACKGROUND OF THE INVENTION The present invention relates to lap dissolve slide projectors.

As used herein the term lap dissolve refers to the gradual fading out of the entire image of one slide projected on'a viewing area while simultaneously developing in substantial spatial correspondence the image of another slide-on the same viewing area.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a lap dissolve slide projector which is an improvement on theslide projector disclosed in my U.S. patent application Ser. No. 51,155 filed 30 June 1970 now U.S. Pat. No. 3,738,741.

It is further an object of the present invention to provide a lap dissolve slide projector capable of previewing a slide before projecting'it on a viewing screen.

In accomplishing these and other objects, there is provided a lap dissolve slide projector arranged to receive and hold a slide at each of two projecting stations. The projector utilizes a polarizing beam splitter and is operable to selectively project the slide loaded in either one of the projecting stations while previewing the slid loaded in the other projecting station.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE drawing is a plan view of a lap dissolve slide projector according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing in more detail, there is shown a slide projector generally indicated by the numeral having a housing 11 shown in broken lines. Mounted centrally at the rear and front, respectively, of the housing 1 1, are a lamp l3 and a projection or objective lens 14. The optical axis of the lens 14 is illustrated aligned with the center axis of the projector 10.

Mounted symmetrically across the center axis of the projector 10 behind the projection lens 14 is a polarizing beam splitter .15. The beam splitter 15 is preferably in the shape of a cube and is a compound prism constructed of two similar prisms l6 and 17 with a multilayer interference film 18 of uniform thickness cemented between their base surfaces. The prisms l6 and 17 illustrated have the shape of isosceles triangles in cross-section and define, respectively, entrance faces 16a, 17a and exit faces 16b, 17b. The prism faces 16a, 17b and 16b, '17a are positioned, respectively, parallel and perpendicular to the center axis of the projector 10. The multilayer material 18 extends at an angle of 45 to the projector center axis. Thereby, the entrance and exit surfaces 16a, 17a, 16b, 17b are all positioned at 45 to the vmultilayer thin film 18.

The indices of refraction of the prisms 17, 16 and of the layers of the multilayer material 18 are preselected to define a Brewster or polarizing angle at the interface of the prisms 16, 17 and multilayer material 18 of approximately 45 for the visible light spectrum. Thereby, the beam splitter 15 is operable to divide an input light beam directed into its entrance faces 16a or 17a along entrance paths at right angles to the planes of the faces 16a, 17a into two equal intensity output beams at right angles to one another which are plane polarized apart. The output beams are transmitted and reflected out the exit faces 16b, 1711 along exit paths perpendicular to the planes of the faces 16b, 17b. The p polarization of an input beam (indicated by crosses) is transmitted by the multilayer film 18 while the s polarization of an input beam (indicated by dots) is reflected thereby. The p polarization of the input beam is the light vibrating parallel to the plane of incident of the input beam, i.e. the plane of the sheet of drawing, while the s polarization is the light vibrating perpendicular to this plane of incidence.

Detailed descriptions of this type of polarizing beam splitter are found in U.S. Pat. No. 2,403,731 issued to S. M. MacNeille and in an article by-A. F. Turner and P. W. Baumeister appearing in Applied Optics, vol. 5, No. 1, Jan. 1966, pp. 69-76.

Positioned on the right and left handed sides of the projector 10 are light directing systems for receiving light from the light source 13, forming the light into a light beam, illuminating a slide, and directing the light illuminating the slide into one of the entrance faces of the beam splitter 15 along an entrance path perpendicular thereto. It is noted that the optical paths of the right and left hand light directing systems are equal in length and a suffix L or R is used to identify optical components in the respective light directing systems.

The right light directing system includes a condensing lens system 25R; an arrangement 26R defining a first projecting station for receiving and holding a slide or the like; and plane mirrors 27R, 28R. The condensing lens system 25R is made up of a lens 29R, plane mirror 30R, and lens 31R and operates to form an unpolarized right converging visible light beam RB which illuminates the slide 32R loaded in the projecting station defined by the slide holding structure 26R. The light beam RB is illustrated by a dashed line and is directed by the mirrors 27R, 28R to the beam splitter entrance face 17a.

The left light directing system is constructed similar to the aforedescribed right light directing system except that the left unpolarized converging visible light beam LB which it forms to illuminate the slide 32L loaded in the slide holding structure 26L is directed by only one plane mirror 27L to the entrance face 16a of the beam splitter 15. The light beam LB is illustrated by a solid line.

Mounted across the light output paths of the beam splitter exit faces 16b, 17b, respectively, are plane polarizer elements 35, 36. The polarizer elements 35, 36 are rotatably mounted in guides 37, 38, respectively, and are interconnected by control means 39, as indicated by broken lines, with their polarization orientations in alignment. The control means 39 may be any suitable mechanical, mechanical-electrical or other arrangement and is selectively operable to rotate both the polarizers 35, 36 together simultaneously in a coordinate action so that the polarizers 35, 36 are maintained in alignment.

The projections lens 14 receives and projects on a viewing area, illustrated as viewing screen 40, the portion of the beam splitter output from exit face 16b which is transmitted by the polarizer 35. Positioned to receive the portion of the beam splitter output from exit face 17b which is transmitted by the polarizer 36 is a slide preview arrangement 50. The preview arrangement 50 is illustrated as being made up of a plane mirror 51, a preview lens 52 and a preview screen 53.

In operation, the slide projector is operable to project the image of the slide 32R on the viewing screen 40 when the plane polarizer 35 is orientated to transmit p polarized light (indicated by crosses). A focussed image of the slide 32L is projected on the screen 40 by the projection lens 14.

With the polarizer 35 orientated to transmit p polarized light, the polarizer 36 is also orientated to transmit p polarized light. Thus, the image of the slide 32L loaded in the projecting station defined by structure 26L is being projected on the preview screen 53. The preview lens 52 operates to form a focussed image of the slide 32L on the preview screen 53. Consequently, the slide 32L may be previewed prior to its projection on the viewing screen 40.

By selectively operating the control means 39 to rotate the orientation of the plane polarizers 35, 36 90 so that they transmit s polarized light (indicated by dots), a lap dissolve transition is made onthe viewing screen 40 from the image of the slide 32R loaded in the projecting station 26R to the image of the slide loaded in the projecting station 26L. It is noted that a lap dissolve transition is also simultaneously made on the preview screen from the image of the slide 32L to the image of the slide 32R. It is noted that since the optical paths of the right and left hand light directing systems are equal that once the projection lens l4-is adjusted to project a focussed image ofa slide in one of the projecting stations 26R,26L on the viewing screen 40 it is also adjusted to project a focussed image of the slide in the other projecting station.

The slide 32R in the projecting station 26R which has already been projected may now be unloaded therefrom and replaced with the next slide to be projected. Once loaded in the projecting station 26R, the image of this next slide to be projected is previewed on the screen 53 since the polarizers 35, 36 are still orientated to transmits polarized light By now selectively operating the control means 39 to again rotate the orientation of the plane polarizers 90, a lap dissolve transition may be made on the viewing screen 40 from the image of the slide 32L loaded in the projecting station 26L to the image of the slide just loaded in the projecting station 26R.

By operating the slide projector 10 in the manner above described, slides may be previewed prior to projection on a viewing screen. Further, the viewing screen 40 need never be blank since the slides may be changed in the projecting station being viewed on the preview screen. Further, since a lap dissolve transition is made from the projection of the image of one slide to another, undesirable changes in light intensity on the viewing screen, such as when the screen goes blank, are eliminated.

It is noted in the operation of the projector 10 that the prism 16 and multilayer material 18 function as one means to s polarize the portion of the light beam LB directed to the polarizer 35 while the prism 17, multilayer material 18 and prism 16 operate as another means to p polarize the portion of the light beam RB directed along the same light exit path to the polarizer 35. The polarizer 35 functions as themeans for controlling which of the beams LB, RB is transmitted to the projecting lens l4.

The preview system of the projector 10 operates on the same principles above described with regard to the projectors projection system.

It is noted that the term slide projector is used herein to encompass all types of equivalent light projecting apparatus.

I claim:

1. A lap dissolve slide projector, comprising:

means defining first and second projecting stations each for receiving and supporting a slide or the like to be projected; means for forming a first unpolarized visible illuminating light beam and a second unpolarized visible illuminating light beam, saidlight beam forming means being positioned to illuminate a slide supported at said first projecting station with said first illuminating beam and to illuminate a slide supported at said second projecting station with said second illuminating beam; polarizing beam splitter means having first and second light entrance paths and a first light exit path, said beam splitter means being operable to generate from an unpolarized visible input light beam received along said first entrance path a first plane polarized output light beam directed along said first exit path, said beam splitter means being operable to generate from an unpolarized visible input light beam received along said second entrance path a second plane polarized output light beam directed along said first exit path which is polarized apart from said first output beam; light directing means positioned with respect to said projecting stations and beam splitter means for directing said first illuminating beam as an input along said first entrance path and said second illuminating beam as an input along said second entrance path; projection lens means for projecting slide images on a selected viewing area, said projection lens means being positioned to receive and project output beams from said beam splitter first exit path; and

first plane polarizing means positioned to intercept output beams from said beam splitter first exit path, the polarization orientation of said first plane polarizing means being selectively changeable between one position aligned with the polarization of said first output beam and another position 90 out of alignment therewith whereby a lap dissolve transition may be made on a selected viewing area from the projection of the image of a slide supported at one of said projecting stations to the image of a slide supported at the other of said projecting stations by rotating said first polarizing means between said one and other positions.

2. The invention defined in claim 1, wherein said polarizing beam splitter is a compound prism made up of two prisms having flat base surfaces with a multilayer material of substantially uniform thickness interposed between said base surfaces, the indices of refraction of said prisms and the layers of said multilayer material having preselected valves to define a selected Brewsters angle at the interfaces of said prisms and multilayer material.

3. The invention defined in claim 2, wherein:

the indices of refraction of said prisms and of the layers of said multilayer material are preselected to define a Brewsters angle of 45; and

5 6 said first exit path is substantially perpendicular to said first exit path being aligned with said second said first entrance path and in alignment with said entrance path, each of said entrance and exit paths second entrance path, each of said entrance and intersecting the plane defined by said multilayer exit paths intersecting the plane defined by said material at approximately a 45 angle. multilayer material at approximately a 45 angle. 5 8. In a lap dissolve slide projector wherein slides in 4. The invention defined in claim 1, wherein: first and second projecting stations are, respectively,

said beam splitter means also has a second light exit illuminated by first and second light beams, said first path, said beam splitter means being operable to and second light beams being each divided into a first divide an unpolarized visible input light beam rebeam portion which is projected from said slide projecceived along said first entrance path into said first l tor by a projection lens and a second beam portion plane polarized output light beam directed along which is not projected from said slide projector, the said .first exit path and a third plane polarized outfirst portions of said first and second light beams being put light beam directed along said second exit path plane polarized 90 apart and said slide projector havwhich is polarized 90 apart from said first output ing a first plane polarizing means for controlling the beam, said beam splitter means further being operl plane of polarization of light projected from said proable to divide an unpolarized visible input light jection lens so that a slide supported in one or the other beam received along said second entrance path of said projecting stations may be selectively projected into said second plane polarized output light beam on a viewing screen by selectively changing the polardirected along said first exit path and a fourth plane ization orientation of said first polarizing means, the polarized output light beam directed along said improvement in combination therewith comprising:

second exit path which is plane polarized 90 apart means for plane polarizing the second portions of from'said second output beam and hence polarized said first and second light beams 90 apart; 90 apart from said third output beam; and includmeans positioned to receive the second portions of ing: said first and second light beams for previewing the means for previewing the image of slide to be proimages of slides supported at said first and second jected, said preview means being positioned to reprojecting stations; and ceive output beams from said beam splitter second second plane polarizing means positioned to control exit path and being operable to form the image of the plane of polarization of light received by said a slide supported in said first or second projecting preview means, the polarization orientation of said station; and second plane polarizing means being selectively second plane polarizing means positioned between changeable between one position whereat a slide said beam splitter preview means to intercept outsupported in said first projecting station is preput beams from said beam splitter second exit path, viewed to another position whereat a slide supthe polarization orientation of said second plane ported in said second projecting station is prepolarizing means being selectively changeable beviewed. tween one position aligned with the polarization of 9. The invention defined in claim 8, including means said fourth output beam and another position 90 for simultaneously changing the polarization orientaout of alignment therewith whereby the previewing tions of said first and second polarizing means, said secof the image ofa slide supported at one or the other 0nd polarizing means being orientated with respect to of said projecting stations may be selectively consaid first polarizing means to transmit to said preview trolled by the orientation of said second plane pomeans the second portion of the one of said light beams larizing means. -not being projected so that the slide in the one of said 5. The invention defined in claim 4, including control projecting stations which is not being projected is premeans for maintaining the orientations of said first and viewed. second polarizing means in phase and coordinately ro- 10. A slide projector, comprising:

tating said polarizing means together selectively bemeans defining first and second projecting stations tween their one and other positions so that when the each for receiving and supporting a slide or the like image of a slide in one of said projecting stations is to be projected;-

means for forming a first unpolarized visible illumibeing projected the image of a slide in the other of said nating light beam and a second unpolarized visible projecting stations is being previewed.

6. The invention defined in claim 5, wherein said poilluminating light beam, said light beam forming larizing beam splitter is a compound prism made up of means being positioned to illuminate a slide suptwo prisms having flat base surfaces with a multilayer ported at said first projecting station with said first material of substantially uniform thickness interposed illuminating beam and to illuminate a slide supbetween said base surfaces, the indices of refraction of ported at said second projecting station with said said prisms and the layers of said multilayer material second illuminating beam; having preselected valves to define a selected Brewpolarizing beam splitter means having first and secsters angle at the interfaces of said prisms and multiond light entrance paths and first and second light layer material. exit paths, said beam splitter means being operable.

7. The invention defined in claim 6, wherein: the indices of refraction of said prisms and of the lay- 11. The invention defined in claim 10, including:

ers of said multilayer material are preselected to means for previewing the image ofa slide to be prodefine a Brewsters angle of 45; and jected, said preview means being positioned to resaid entrance paths are substantially perpendicular to ceive output beams from said beam splitter second each other and preview said exit paths are substanexit path and being operable to form the image of tially perpendicular to each other, said second exit a slide supported in said first or second projecting path being aligned with said first entrance path and station; and wherein:

said means for plane polarizing light also intercepts the component of said first light beam made up of light polarized 90 apart from said predetermined orientation and thecomponent of said second light beam made up of light polarized at said determined orientation whereby change of the polarization orientation of said plane polarizing means between alignment with said determined orientation and 90 out of alignment therewith causes the image of the one of the slides in said projecting stations not being projected to be previewed.

12. The invention defined in claim 11, wherein said polarizing beam splitter means is a compound prism made up of two prisms having flat base surfaces with a multilayer material of substantially uniform thickness interposed between said base surfaces and defines a selected Brewsters angle for the visible light spectrum at the interfaces of said prisms and multilayer material.

13. The invention defined in claim 12 wherein:

said polarizing beam splitter means defines a Brewsters angle of approximately 45 for the visible light spectrum; and to direct visible light received along said first input path polarized in a determined plane of orientation out said first exit path and polarized in a plane 90 apart from said determined orientation out of said second exit path, said beam splitter means further being operable to direct visible light received along said second input path plane polarized 90 apart from said determined orientation out said first exit path and plane polarized in said determined orientation out said second exit path;

light directing means positioned with respect to said projecting stations and beam splitter means for directing said first illuminating beam as an input along said first entrance path and said second illuminating beam as an input along said second entrance path;

projection lens means for projecting slide images on a selected viewing area, said projection lens means being positioned to receive and project output beams from said beam splitter first exit path; and

means for plane polarizing light positioned to intercept the component of said first light beam made up of light polarized at said determined orientation and the component of said second light beam made up of light polarized apart from said determined orientation, the polarization orientation of said plane polarizing means being changeable between alignment with said determined plane of polarization orientation and 90 out of alignment therewith whereby transition may be made on a selected viewing area from the projection of the image of a slide supported at one of said projecting stations to the image of a slide supported at the other by so changing the polarization orientation of said plane polarizing means each of said entrance and exit paths intersect the plane defined by said multilayer material at approximately a 45 angle.

UNITED STATES PATENT armor CERTIFICATE OF QGRREMWN Patent No. 5 ,:Z22,Q22 Dated 19 Februarv 197d Inventofls) THOMAS A. FOURNIE It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In column 5, line 52 should read said beam splitter interior moan ou p on and line 65 should read --=-each other and said exit paths are substan- Claim 10, printed at column 6, lines 45-59, should read:

--1 0.A slide projector, comprising: 7

means defining first and second projecting stations each for receiving and supporting a slide or the like to be projected;

means for forming a first unpolarized visible illuminating light beam and a second unpolarized visible illuminating light beam, said light beam forming means being positioned to illuminate a slide supported at said first projecting station with said first illuminating beam and to illuminate a slide supported at said second projecting station with said second illuminating beam;

polarizing beam splitter means having first and second light entrance paths and first and second light exit paths, said 7 beam splitter means being operable to direct visible light received along said first input path polarized in a determined plane of orientation out said first exit path and polarized in a plane 90 apart from said determined orientation out said second exit path, said beam splitter means further operable to direct visible light received along said second input path plane polarized 90f: apart from said determined orientation out said first exit path and plane polarized in said determined orientation out said second exit path;

page 1 of 5 FORM P9405) 7 I USCOMM-DC wan-Poo 9 ".5. GOVERNMENT PRINTING OFFICE; I959 0-365-J3M UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIGN Pat nt No- 5,792,922 Dated 19 February 1974 Inventor) THOMAS A, FOQEIE light directing means positioned with respect to said pro-- Jecting stations and beam splitter means for directing said first illuminating beam as an input along said first entrance path and said second illuminating beam as an input along said second entrance path;

projection lens means for projecting. slide images on a selected viewing area, said projection lens means being positioned to receive and project output beams from said beam splitter first exi Path; and p means for plane polarizing light positioned to intercept the component oi said first light beam made up of light polarized at said determined orientation and the component of said secand light beam made up of light polarized 90 apart from said determined orientation, the polarization orientation of said plane polarizing means being changeable between alignment with said determined plane of polarization orientation and 90 out oi alignment therewith whereby transition may be made on a selected viewing area from the projection of the image of a slide supported at one of said projecting stations to the image of a slide supported at the other by so changing the polarization orientation of said Plane polarizing means.

In column 7, at line 3 "predetermined" should read -determined-.

' page 2 of 5 OR Podoso (o-69) USCOMM-DC 60376-P69 I ".5. GOVERNMENT PRINTING OFFICE l9 0-366-534,

UNITED STATES PATENT OFFICE CERTIFICATE OF COEQTEN Patent No. 3,792,922 Dated 19 February 1974 Inventora) THOMAS A'. FOURNIE end-Gleam Claim 13, printed at column 7 line 19 column 8 line 30, should read: --13.The invention defined in claim 12 wherein: said polarizing beam splitter means defines a Brewsters e angle of approximately 459 for the visible light spectrum; each-of said entrance and exit paths intersect the plane defined by said multilayer material at approximately-a 45 angle.--

Signed sealed this 5th day of November 197%,

(SEAL) Attest: I

MCCOY M. GIBSON JR. 0. MARSHALL DANN Attesting Officer A Commissioner of Patents FORM Podoso (o-6s) l USCOMM-DC MUG-P69 1 us. GOVERNMENT PRINTING omc: use o-ass-ssa. 

1. A lap dissolve slide projector, comprising: means defining first and second projecting stations each for receiving and supporting a slide or the like to be projected; means for forming a first unpolarized visible illuminating light beam and a second unpolarized visible illuminating light beam, said light beam forming means being positioned to illuminate a slide supported at said first projecting station with said first illuminating beam and tO illuminate a slide supported at said second projecting station with said second illuminating beam; polarizing beam splitter means having first and second light entrance paths and a first light exit path, said beam splitter means being operable to generate from an unpolarized visible input light beam received along said first entrance path a first plane polarized output light beam directed along said first exit path, said beam splitter means being operable to generate from an unpolarized visible input light beam received along said second entrance path a second plane polarized output light beam directed along said first exit path which is polarized 90* apart from said first output beam; light directing means positioned with respect to said projecting stations and beam splitter means for directing said first illuminating beam as an input along said first entrance path and said second illuminating beam as an input along said second entrance path; projection lens means for projecting slide images on a selected viewing area, said projection lens means being positioned to receive and project output beams from said beam splitter first exit path; and first plane polarizing means positioned to intercept output beams from said beam splitter first exit path, the polarization orientation of said first plane polarizing means being selectively changeable between one position aligned with the polarization of said first output beam and another position 90* out of alignment therewith whereby a lap dissolve transition may be made on a selected viewing area from the projection of the image of a slide supported at one of said projecting stations to the image of a slide supported at the other of said projecting stations by rotating said first polarizing means between said one and other positions.
 2. The invention defined in claim 1, wherein said polarizing beam splitter is a compound prism made up of two prisms having flat base surfaces with a multilayer material of substantially uniform thickness interposed between said base surfaces, the indices of refraction of said prisms and the layers of said multilayer material having preselected valves to define a selected Brewster''s angle at the interfaces of said prisms and multilayer material.
 3. The invention defined in claim 2, wherein: the indices of refraction of said prisms and of the layers of said multilayer material are preselected to define a Brewster''s angle of 45*; and said first exit path is substantially perpendicular to said first entrance path and in alignment with said second entrance path, each of said entrance and exit paths intersecting the plane defined by said multilayer material at approximately a 45* angle.
 4. The invention defined in claim 1, wherein: said beam splitter means also has a second light exit path, said beam splitter means being operable to divide an unpolarized visible input light beam received along said first entrance path into said first plane polarized output light beam directed along said first exit path and a third plane polarized output light beam directed along said second exit path which is polarized 90* apart from said first output beam, said beam splitter means further being operable to divide an unpolarized visible input light beam received along said second entrance path into said second plane polarized output light beam directed along said first exit path and a fourth plane polarized output light beam directed along said second exit path which is plane polarized 90* apart from said second output beam and hence polarized 90* apart from said third output beam; and including: means for previewing the image of slide to be projected, said preview means being positioned to receive output beams from said beam splitter second exit path and being operable to form the image of a slide supported in said first or second projecting station; and second plane polarizing means posiTioned between said beam splitter preview means to intercept output beams from said beam splitter second exit path, the polarization orientation of said second plane polarizing means being selectively changeable between one position aligned with the polarization of said fourth output beam and another position 90* out of alignment therewith whereby the previewing of the image of a slide supported at one or the other of said projecting stations may be selectively controlled by the orientation of said second plane polarizing means.
 5. The invention defined in claim 4, including control means for maintaining the orientations of said first and second polarizing means in phase and coordinately rotating said polarizing means together selectively between their one and other positions so that when the image of a slide in one of said projecting stations is being projected the image of a slide in the other of said projecting stations is being previewed.
 6. The invention defined in claim 5, wherein said polarizing beam splitter is a compound prism made up of two prisms having flat base surfaces with a multilayer material of substantially uniform thickness interposed between said base surfaces, the indices of refraction of said prisms and the layers of said multilayer material having preselected valves to define a selected Brewster''s angle at the interfaces of said prisms and multilayer material.
 7. The invention defined in claim 6, wherein: the indices of refraction of said prisms and of the layers of said multilayer material are preselected to define a Brewster''s angle of 45*; and said entrance paths are substantially perpendicular to each other and said exit paths are substantially perpendicular to each other, said second exit path being aligned with said first entrance path and said first exit path being aligned with said second entrance path, each of said entrance and exit paths intersecting the plane defined by said multilayer material at approximately a 45* angle.
 8. In a lap dissolve slide projector wherein slides in first and second projecting stations are, respectively, illuminated by first and second light beams, said first and second light beams being each divided into a first beam portion which is projected from said slide projector by a projection lens and a second beam portion which is not projected from said slide projector, the first portions of said first and second light beams being plane polarized 90* apart and said slide projector having a first plane polarizing means for controlling the plane of polarization of light projected from said projection lens so that a slide supported in one or the other of said projecting stations may be selectively projected on a viewing screen by selectively changing the polarization orientation of said first polarizing means, the improvement in combination therewith comprising: means for plane polarizing the second portions of said first and second light beams 90* apart; means positioned to receive the second portions of said first and second light beams for previewing the images of slides supported at said first and second projecting stations; and second plane polarizing means positioned to control the plane of polarization of light received by said preview means, the polarization orientation of said second plane polarizing means being selectively changeable between one position whereat a slide supported in said first projecting station is previewed to another position whereat a slide supported in said second projecting station is previewed.
 9. The invention defined in claim 8, including means for simultaneously changing the polarization orientations of said first and second polarizing means, said second polarizing means being orientated with respect to said first polarizing means to transmit to said preview means the second portion of the one of said light beams not being projected so that the slide in the one of said projectinG stations which is not being projected is previewed.
 10. A slide projector, comprising: means defining first and second projecting stations each for receiving and supporting a slide or the like to be projected; means for forming a first unpolarized visible illuminating light beam and a second unpolarized visible illuminating light beam, said light beam forming means being positioned to illuminate a slide supported at said first projecting station with said first illuminating beam and to illuminate a slide supported at said second projecting station with said second illuminating beam; polarizing beam splitter means having first and second light entrance paths and first and second light exit paths, said beam splitter means being operable. to direct visible light received along said first input path polarized in a determined plane of orientation out said first exit path and polarized in a plane 90* apart from said determined orientation out said second exit path, said beam splitter means further being operable to direct visible light received along said second input path plane polarized 90* apart from said determined orientation out said first exit path and plane polarized in said determined orientation out said second exit path; light directing means positioned with respect to said projecting stations and beam splitter means for directing said first illuminating beam as an input along said first entrance path and said second illuminating beam as an input along said second entrance path; projection lens means for projecting slide images on a selected viewing area, said projection lens means being positioned to receive and project output beams from said beam splitter first exit path; and means for plane polarizing light positioned to intercept the component of said first light beam made up of light polarized at said determined orientation and the component of said second light beam made up of light polarized 90* apart from said determined orientation, the polarization orientation of said plane polarizing means being changeable between alignment with said determined plane of polarization orientation and 90* out of alignment therewith whereby transition may be made on a selected viewing area from the projection of the image of a slide supported at one of said projecting stations to the image of a slide supported at the other by so changing the polarization orientation of said plane polarizing means.
 11. The invention defined in claim 10, including: means for previewing the image of a slide to be projected, said preview means being positioned to receive output beams from said beam splitter second exit path and being operable to form the image of a slide supported in said first or second projecting station; and wherein: said means for plane polarizing light also intercepts the component of said first light beam made up of light polarized 90* apart from said determined orientation and the component of said second light beam made up of light polarized at said determined orientation whereby change of the polarization orientation of said plane polarizing means between alignment with said determined orientation and 90* out of alignment therewith causes the image of the one of the slides in said projecting stations not being projected to be previewed.
 12. The invention defined in claim 11, wherein said polarizing beam splitter means is a compound prism made up of two prisms having flat base surfaces with a multilayer material of substantially uniform thickness interposed between said base surfaces and defines a selected Brewster''s angle for the visible light spectrum at the interfaces of said prisms and multilayer material.
 13. The invention defined in claim 12 wherein: said polarizing beam splitter means defines a Brewster''s angle of approximately 45* for the visible light spectrum; and each of said entrance and exit paths intersect the plane defineD by said multilayer material at approximately a 45* angle. 